1. Field of the Invention
This invention relates to a thin-film device which comprises a thin-film deposited on a substrate by evaporation, sputtering, etc., to attain desired electrical characteristics, and, more in particular, to a thin-film bipolar device which may be constructed by deposition of thin-films in a desired pattern by a variety of chemical, mechanical, or high-vacuum evaporation techniques with ease.
2. Description of the Prior Art
It has been common practice to employ the thin-film technology to mainly construct passive elements such as resistors and capacitors, or interconnections. And, as the state of technology, when one mentions a thin-film circuit, it generally means the circuit in which the passive elements and conductors are produced as films on a substrate. If it is required to provide active elements such as diodes and transistors as well, discrete or monolithic semiconductor devices must be combined with the thin-film devices to form a hybrid circuit.
Semiconductor devices have been widely used and they are increasingly used even now. One of the advantages of the semiconductor devices is that they may be formed on a small-sized single-crystal chip of silicon. Thus, the semiconductor technology is particularly useful in constructing a high density device such as an integrated circuit device, containing both active and passive elements and their interconnections. It should be noted, however, that since there is a maximum size limit for the silicon wafer, disadvantages are sometimes brought about due to incapability of direct applications. This is particulary true for an optical reader including an array of photoconductive elements required to scan the substantial length of an original.
For such an application to an optical reader, bipolar devices are required rather than MOS devices. For example, a phototransistor may be used as a reading element of the array. Such a phototransistor is a bipolar transistor which includes emitter, base and collector regions with two P-N junctions at the boundaries between the respective adjacent regions. It is true that bipolar transistors having a PNP or NPN structure may be manufactured on a single-crystal chip of silicon or germanium in accordance with the wellknown semiconductor device manufacturing technology. However, these single-crystal bipolar transistors require very fine and carefully conditioned processing which tends to increase the manufacturing cost and to lower the yield. Moreover, since there is a maximum size limit for this type of bipolar transistors, an unacceptably large number must be provided, or, alternatively, an additional interfacing circuit must be provided in certain applications which involve a relatively large area or a sizable length. If many transistors of this type were to be disposed along a line, then it would bring about the problem of non-uniform characteristics one from another.